The present invention relates to data communications and, more particularly, to high speed data communications over a four-wire subscriber line.
The present invention is concerned with data communications over four-wire subscriber lines. Subscriber line communication equipment typically operates over switched two-wire connections that are limited to half-duplex or asymmetrical communication, i.e. transmission, or high-speed transmission, in only one direction at a time between endpoints of the connection. Full-duplex transmission, i.e. simultaneous transmission in both directions between endpoints, is possible using four-wire subscribe lines. Four-wire subscriber lines, also known as private leased lines (PLLs), are non-switched lines through the public telephone network that are typically leased by an end user, or subscriber, for a dedicated communication link between two endpoints. Each four-wire subscriber line is composed of two pairs of conductors, one pair for each direction of communication. A modem adapted for use with four-wire lines terminates each end of the communications link.
FIG. 1 is an architecture diagram illustrating an example of a conventional four-wire communications link 10 between a subscriber""s end points. Modems 22 and 42 are customer premises equipment (CPE) located at the customer""s facilities and connected to the customer""s data termination equipment (DTE) devices 20 and 40, respectively. A four-wire subscriber line connects modems 22 and 42 through the PSTN. A first pair of lines 32A and 32B provides the transmit pair from transmit terminals (TX) for modem 22 to receive terminals RX for modem 42. Similarly, a second pair of lines 34A and 34B provides the transmit pair from transmit terminals (TX) of modem 42 to receive terminals (RX) of modem 22.
Modems 22 and 42 are data communications equipment (DCE) and have a series of lines for interfacing with their respective DTE devices. In the example of FIG. 1, the lines are specified by the RS-232C/V.24 standard. A transmit data (TD) line of each modem receives data from a data output terminal of the corresponding DTE for transmission to the modem at the other end of the communications link. Likewise, data received by each modem over the communications link is output to a data input terminal of the corresponding DTE over the receive data (RD) line. A data transmit control terminal of the DTE is connected to a request-to-send (RTS) line of the modem and signals the modem that the DTE is about to send data to the modem over the TD line. For example, when modem 22 receives an active signal from DTE device 20 connected to its RTS line, modem 22 transmits a carrier signal to modem 42 over a first pair of lines 32A and 32B. Once the connection with the modem 42 is established, modem 22 will modulate the carrier signal on the first pair of lines 32A and 32B with data received from DTE device 20 on the TD line of modem 22. Modem 42 receives the modulated signal transmitted on the first pair of lines 32A and 32B, demodulates the received signal to obtain the data signal, and outputs the received data over its RD line to DTE device 40.
Likewise, when modem 22 senses a carrier signal transmitted on a second pair of lines 34A and 34B, it activates a carrier detect (CD), line connected to a data input control terminal of DTE device 20. Modem 22 demodulates the signal received over the second pair of lines 34A and 34B to obtain the data signal from modem,42 and outputs the received data over its RD line connected to a data input terminal of DTE device 20.
Some transmission protocols, such as the International Telephone Union (ITU) V.34 standard, are configured to accommodate full-duplex transmission, i.e. simultaneous transmission in both directions, over a four-wire subscriber line. However, the maximum data rate for V.34 is limited to 33.6 kilobits-per-second (Kbps). Many higher speed protocols, such as the V.90 standard, which is capable of a 56 Kbps transmission rate, are not configured to accommodate full-duplex transmission.
Therefore, the need remains for an approach that provides for high speed transmission over a four-wire subscriber line.
In accordance with preferred embodiments of the present invention, some of the problems associated with data transmission over four-wire subscriber lines in the prior art are overcome.
An embodiment of a communication system for transmitting data over a four-wire subscriber line, according to the present invention, includes a first data terminal device having a data output control terminal, a data output terminal, a data input control terminal and a data input terminal. The system also includes a first modem having a receive section and a transmit section. The receive section of the first modem has first and second line input terminals for coupling to a first pair of wires of the four-wire subscriber line, a receive control output terminal coupled to the data input control terminal of the first data terminal device for outputting a signal to the first data terminal device when a first encoded data signal is present on the first pair of wires, and a receive data output terminal coupled to the data input terminal of the first data terminal device. The receive section of the first modem is configured to decode the first encoded data signal present on the first pair of wires according to a predetermined data transmission protocol in order to generate a first unencoded data signal at the receive data output terminal. The transmit section of the first modem has first and second line output terminals for coupling to a second pair of wires of the four-wire subscriber line, a transmit control input terminal coupled to the data output control terminal of the first data terminal device for receiving a signal from the first data terminal device that the first data terminal device is going to output a second unencoded data signal on the data output terminal of the first data terminal device, and a transmit data output terminal coupled to the data output terminal of the first data terminal device. The transmit section is configured to encode the second unencoded data signal received on the transmit data output terminal according to the predetermined data transmission protocol in order to transmit a second encoded data signal on the second pair of wires.
An embodiment of a method for high-speed full-duplex communication over a four-wire subscriber line, according to the present invention, provides for receiving a first unencoded data signal from a first data terminal device, encoding the first unencoded data signal according to a predetermined data transmission protocol to obtain a first encoded data signal, and transmitting the first encoded data signal over a first pair of wires of the four-wire subscriber line. The method also calls for simultaneously receiving a second encoded data signal over a second pair of wires of the four-wire subscriber line, decoding the second encoded data signal according to the predetermined data transmission protocol to obtain a second data signal, and outputting the second data signal to the first data terminal device.